Gas Target
| Version | Author | Description | Date |
|---|---|---|---|
| 1.0 | D. Hutcheon | Initial Document | 27 September 2002 |
| 1.1 | D. Hutcheon | Hidden valves,shack | 4 April 2007 |
| 1.2 | D. Hutcheon | Leak checking | 27 April 2007 |
| 1.3 | D. Hutcheon | Decadal update | 22 January 2019 |
| 1.4 | A. Lennarz | Transferred to wiki | June 25th, 2025 |
Safety with hydrogen as the target gas
The following is a brief outline of safety considerations. For more details, consult the DRAGON Safety Report.
The strategy for safe operation with hydrogen gas is:
- avoid formation of an explosive mixture of air and hydrogen
- avoid ignition sources at pressures where combustion may take place
Normal operation
When the zeolite cleaning trap is in operation, there is a large inventory of "invisible" hydrogen adsorbed on the zeolite molecular sieve. For a trap inlet pressure (CMTRIN) of 45 Torr the adsorbed hydrogen amounts to 5 litres at STP, while the "free" recirculating hydrogen may be only 15% of this amount. The heat of combustion of this total amount of hydrogen is approximately 50 kJ - enough to raise 5 tonnes a height of 1 m.
IT IS ESSENTIAL THAT THE TARGET OPERATOR KNOWS WHETHER THE TRAP IS COLD AND WHETHER IT IS "LOADED" WITH HYDROGEN.
The correct response to the sonalert alarm from the LN2 automatic filler or from the EPICS system depends upon this knowledge. Therefore:
ANY SIGNIFICANT CHANGE IN THE INVENTORY OF HYDROGEN IN THE CLEANING TRAP MUST BE ACCOMPANIED BY AN ENTRY IN THE DRAGON “Equipment Status” elog (https://elog.triumf.ca/Dragon/}.
The entry should indicate trap status (warm/cold), estimated hydrogen inventory.. The 6-litre Buffer Tank should be used to control the amount of gas loaded into the trap. The number of 50-Torr "loads" from the Inlet Buffer Tank should be tallied during an initial fill of the trap. Any large deviation from the expected 15-20 "loads" to reach approx. 45 Torr pressure must be investigated and the problem fixed.
(During tuning for a new beam energy it is common for ISAC Operations to request that gas be removed from the DRAGON target. Each time that this is done, some of the "free" circulating hydrogen is lost. It is convenient to replenish the supply by adding 1 or 2 "loads" of gas through the Inlet Buffer Tank when the target cell is again filled with recirculating gas. This is considered a routine operation which need not be reported in the Equipment Status elog)
The purpose of the cleaning trap is to remove air and other impurities from the recirculating hydrogen gas. Thus, when the trap does its job it is almost impossible to detect any small leak of air into the target system. The implications for target operation with a trap are:
- A cold trap which has been loaded with hydrogen should be treated as though it contains air also, in a potentially explosive mixture. An exhaust fan must be running whenever a cold trap contains hydrogen, to allow safe dilution of hydrogen in the event of an unanticipated release from the trap through the pressure relief valve. Normal procedure when the trap is being warmed up is to pump on it with the roughing pump, which vents to the high-air-flow exhaust line.
- Before cooling the trap, it must be checked that any air leaks are less than could amount to 1.5 atm-litre over a 2-week period (assuming a trap will be kept cold no longer than 2 weeks at a time). Possible sources include leaks in the differential pumping stages, in the trap itself or in the buffer tank and gas supply lines. The leak rates should be measured by pumping down the system, then isolating it with the Roots blowers continuing to run; the gauge CMTRIN connects to an effective volume of 12 litres, from which the quantity of gas (leaking or outgassing) may be calculated using an observed rate of increase in CMTRIN. Other sources are HEBT or the DRAGON separator when isolation valves HEBT2:IV8 or DRA:IV11 are open. At an estimated 1000 litre/sec conductance in the beam pipes, the sum of partial pressures of air upstream and downstream of the target should be less than 1E-6 Torr when there is no gas added to the trap.
- Failure of the automatic LN2 filling system or failure of the primary exhaust fan must trigger immediate action, either to fix the fault or to begin controlled warmup of the trap.
Unattended gas target, trap containing absorbed hydrogen
At times it may be desirable for the DRAGON gas target operator to leave the target unattended for a period of many hours. This may be done, provided it is arranged that someone else (e.g. an ISAC Operator) checks the Trap status via Epics at least once every two hours. Of greatest concern is failure to keep the trap cold, for example if an LN2 dewar goes empty or the automatic filling system fails.
Protection of equipment
Equipment upstream of the gas target (HEBT) or downstream (Separator) is protected against excessive gas loads or shock waves from the DRAGON target via the interlock conditions on isolation valves HEBT2:IV8 and DRA:IV11. These require satisfactory readings (low pressure and no error condition) in ion gauges on either side of the valve. Equally, the interlocks protect the gas target against HEBT and the Separator. The interlocks on IV8 and IV11 should never be bypassed, nor should ion gauges IGU3 or IGD4 be operated in a mode where they give spurious indication of high vacuum, due to malfunction.
The Roots blower which operates at the highest pressure does not have any internal bypass. It should not be operated with the pressure at CMTRIN greater than 60 Torr.
Turbo-molecular pumps may be damaged if subjected to a burst of air (or other gas) when they are rotating at high speed. When venting the system after turbos have been on, follow the 'spin-down' operations given in the sections on "venting" below.
Gases of high molecular weight are less efficient than hydrogen or helium at cooling turbo pump rotors. Circulation of such gases at multi-Torr pressures has led to overheating and destruction of turbo pumps.
General description of the gas target system
Gas supply shack
Gas for the DRAGON windowless target, as well as the isobutane for its ion chamber detector, is supplied from a small building just outside the ISAC-1 hall to the east side ("the shack"). The general TRIUMF access key 2T85 will open the locked door. Gas is supplied from cylinders, passes through a flow gauge and then is conducted by stainless steel tubing along the inside north wall of ISAC-1 hall and finally south to DRAGON. Return lines provide for venting of gas at the shack: this happens for the isobutane of the ion chamber, but the target has its own separate venting line to roof fans.
A member of the Detector Facility Group is responsible for supplying gas cylinders and for general operations within the gas shack. One of the group should be informed when it is planned to start gas flow for an experiment or to shut off the gas at the end of an experiment.
Permanent metal lines connect hydrogen, helium and isobutane cylinders plus backup cylinders to a manifold located near the centre of the shack.
The manifold has clearly labelled flow meters and lines for the DRAGON target and isobutane detector. The experimenter should not disconnect or connect lines. The choice of hydrogen or helium gas is made by a valve below the target flow meter, together with opening the appropriate supply valve (between the manifold structure and the overhead cable tray) and closing the valve of the unwanted gas. Do not turn off gas at the cylinder.
Note that isobutane is stored in the cylinders as a liquid, not a high-pressure gas, so that the fullness of a cylinder is monitored by its weight. It is arranged that the backup isobutane supply will automatically come into service when the primary cylinder becomes empty.
Windowless gas target
A system of pipes and valves connects the roughing pump to any of several volumes: an inlet buffer tank; a cleaning trap; differential pumping stages; a Helium-3 handling system [currently (2019) not in use]. The valve interlock logic is designed to ensure that only one of these volumes is roughed down at a time.
Other connections permit: recirculation of gas through the cleaning trap or bypassing the trap; addition of gas from the buffer tank to the recirculation volume; controlled removal of gas from the recirculation system via an outlet buffer volume (3 litres). Pressure relief valves allow discharge from the cleaning trap or from the differential pumping volume into the roughing-pump exhaust line if pressures go above ambient air pressure.
The valves, pumps and gauges are controlled by EPICS interface to the PLC (programmable logic controller) of the DRAGON gas target and vacuum system. A right-button mouse click on the "Vacuum" button of the DRAGON EPICS menu will show the various pages associated with the gas target and separator vacuum. Figure 1 shows the page that gives overall control of the gas target Recirculation system.
Figure 1: EPICS page for the gas target Recirculation system. Control of the Roots blowers and ion gauges is on the page "Gas target".
In the most common mode of operation, recirculation mode, hydrogen gas is continuously injected into a windowless target cell. The gas which escapes through the cell apertures is recaptured, compressed, sent through a cleaning trap, and recirculated back into the target cell. Typical pressures are 4.5 Torr in the cell, 0.3 Torr in the manifold around the cell, 30-50 Torr after compression and in the trap, and 10-6 Torr at the HEBT and DRAGON Separator ends of the differential pumping stages.
Another operating mode also recirculates the gas, but sends it back directly in a line which bypasses the cleaning trap. This would be used if a special mixture of hydrogen "spiked" with heavy inert gas is to be used for normalization purposes.
Finally, in flow-through mode, gas might be pumped away by the roughing pump, with no recirculation. This last mode would likely be used only for limited periods, as it consumes gas at a high rate (approximately 250 Torr-litres/second for 4.5 Torr central cell pressure).
The cell pressure control is by a combination of the pressure of recirculating gas and the opening of a flow valve "GCV1" ( located under the "green table" surface of the gas target stand). Fine control of cell pressure may be done by opening/closing the fine flow control valve (GCV1), by pushbutton operation at a panel in the gas target rack. Operation at cell pressures above approx. 6 Torr requires opening of a manual valve which is in parallel with GCV1 (the "red-handled valve" located beside GCV1).
When the cleaning trap is to be used, addition of gas to the target system requires a considerable time: the trap material, X-13 zeolite molecular sieve, adsorbs hydrogen through mechanisms having rather different time constants. The first, rapid, adsorption takes place within seconds but the second mechanism has a time scale of 10-20 minutes. Loading of the trap is accomplished by isolating the Buffer Tank, filling it to 50 Torr from the supply cylinder, isolating it from the supply cylinder, opening it to the recirculation system, allowing gas to flow into the trap, and again isolating the Buffer Tank. By this procedure it is possible to keep track of the quantity of gas admitted to the system (and residing mainly in the trap).
During an experiment it may be necessary to reduce the inventory of gas in the trap/recirculation system, for example because the central cell pressure is to be reduced by a large factor. The inventory may be reduced in a controlled way by successively filling and emptying a known volume (3 litres) located between valves GOT1V and GOT2V. One "glug" through this volume should reduce the pressure at TRIN by about 10% when recirculating gas without the cleaning trap.